Work machines, such as off-highway trucks, are often loaded with heavy payloads and thereafter required to operate over varying terrain. One particularly common scenario is when an off-highway truck is loaded with rock or other material and is thereafter required to traverse a downward grade. Such downward grades can often be relatively steep. For example, travel ways in or out of mining operations commonly have 10% downward grades associated therewith.
On such a steep downward grade, the off-highway truck is retarded in order to keep the truck from overspeeding. In particular, the off-highway truck's brake system is utilized to slow advancement of the truck as it traverses the downward grade. In addition to preventing overspeeding of the truck itself, retarding of the truck is utilized to prevent mechanical damage to the truck. For example, as the off-highway truck advances down the downward grade, engine speed of the truck's engine typically increases due to increasing ground speed of the truck. Hence, if ground speed of the off-highway truck is not retarded to a certain level, the engine may overspeed thereby potentially causing damage thereto. However, during retarding of the off-highway truck, it is desirable to operate the engine at a relatively high engine speed which is within the operating parameters of the engine. This is true since the radiator fan associated with the engine's radiator is driven by the engine. Hence, it is desirable to operate the engine at an engine speed which causes the radiator fan to turn at or near its maximum speed so as to most effectively cool the engine coolant advancing through the radiator.
It should be appreciated that the brake system of the off-highway truck generates relatively large amounts of heat during retarding the truck. In particular, the disc brake assemblies associated with the truck generate relatively large amounts of heat due to the relatively heavy use thereof during retarding of the truck. Hence, a number of off-highway trucks have heretofore been designed with fluid-cooled brake systems. Such fluid-cooled brake systems circulate a brake coolant through a number of coolers and thereafter into contact with the discs of the disc brake assemblies thereby cooling the discs during operation of the off-highway truck. It should be appreciated that it is desirable to operate the circulation pumps associated with the brake cooling system at or near their maximum pump speeds so as to most effectively cool the brake coolant advancing through the brake cooling system.
In order to operate the radiator fan and the circulation pumps of the brake cooling system at or near their maximum speeds, retarding systems which have heretofore been designed typically operate the engine at a predetermined engine speed in order to cause the radiator fan and the circulation pumps of the brake cooling system to be operated at or near their maximum speeds. For example, retarding systems which have heretofore been designed which operate the engine at an engine speed of 1950 revolutions-per-minute (RPM) thereby setting ground speed of the truck in a given gear. Such operation of the engine maintains operation of the radiator fan and the circulation pumps of the brake cooling system at or near their maximum speeds thereby preventing the respective speeds of the radiator fan and the circulation pumps of the brake cooling system from fluctuating due to changes in the ground speed of the off-highway truck.
However, such operation of the engine has a number of drawbacks associated therewith. For example, when operated in such a manner, the truck has a limited number of "set points" in which it may be operated during retarding thereof. In particular, the truck may be operated at the predetermined engine speed (e.g. 1950 RPM) in each of the gears associated with the truck's transmission, but not at any other set points therebetween. For example, the truck may be operated at 1950 RPM in second gear or 1950 RPM in third gear, but could not be operated at 1500 RPM in third gear. Such operation may undesirably reduce the work efficiency of the truck. For instance, as the off-highway truck advances (in a retarding manner) down a downward grade with the truck's transmission in third gear, the truck's engine speed may have a tendency to increase beyond the predetermined engine speed (1950 RPM) due to ground speed of the truck. The brakes are then applied to keep the engine from overspeeding above 1950 RPM. However, the energy put into the cooling system by application of the brakes may be too high for the cooling system to dissipate. Hence, the transmission of the truck must be downshifted to second gear in order to prevent overheating. It should be appreciated that although overheating is alleviated when the transmission is downshifted to second gear, ground speed of the truck is slowed to an undesirable pace which reduces the work efficiency of the truck.
What is needed therefore is a method and apparatus for retarding a work machine which overcomes one or more of the above-mentioned drawbacks.